It is not understood why hepatitis C virus (HCV) infection persists despite a broad cellular and humoral immune response. Because HCV does not cause cirrhosis or liver cancer in the 10-15 percent who clear infection, it is important not only to understand why viral clearance occurs but also to recognize the outcome clinically. Perhaps the largest obstacle to understanding HCV clearance is the rare recognition of acute infection in humans. We have recently overcome this limitation by anticipating infection in those exposed to HCV. Using the resulting repository of sera and peripheral blood mononuclear cells, we plan to examine when viral clearance occurs and identify the predictive clinical features. A grant was recently awarded to examine the evolution of the HC quasispecies in acutely infected subjects. Presently, we propose correlating the temporal changes in the viral quasispecies of these individuals with the emergence of CD8+ lymphocytes that recognize the resolving sequences. The central hypothesis underlying this investigation is that HCV clearance occurs in association with a cytotoxic CD8+ lymphocyte (CTL) responses to conserved epitopes. CTL responses are important in the pathogenesis of many viruses and have been detected in persons with persistent HCV infection. However, the extraordinary antigenic diversity of HCV may limit the relevance of CD8+ lymphocyte detection systems that rely on hypothetical peptide targets. Thus we will use our cDNA library of acute infection to develop an ex vivo instrument that measures the CD8+ lymphocyte response to the sequence of the variant cleared in vivo. Expression of resolving sequences in an autologous cell line further strengthens the in vivo relevance of the approach. The cytotoxicity (and escape from) CD8+ lymphocyte responses will then be proven by evaluating putative CTL clones in a system expressing the specific variant sequence, as planned in Project 1. In addition, the peripheral CD8+ lymphocyte responses will be correlated with intrahepatic responses of experimentally infected chimpanzees, in collaboration with Project 2. This research is the natural extension of team's work on HCV clearance including the ongoing studies of viral sequence evaluation in acute infection. The capability to effectively examine the CD8+ lymphocyte response is enhance substantially by collaboration with Drs. Siliaciano and Schneck and the planned work of the other Projects. A high likelihood of success is anticipated given the unique infection from humans and chimpanzees, matched library of viral sequences, and the experience of the investigative team.